Poles Apart: A Tale of Two Oceans

At the opposite ends of the earth lie the Arctic and Southern Oceans. Don’t know much about them? You’re in good company. These two bodies of water are often overlooked as we focus on the more familiar Atlantic, Pacific, and Indian Oceans. Yet they are surprisingly different from each other, are home to a wide variety of species, and play important roles in the polar regions’ and our earth’s climates.

THE ARCTIC OCEAN: WATER SURROUNDED BY LAND

The Arctic Ocean lies almost entirely within the Arctic Circle (66 degrees N latitude). It is the smallest and shallowest of earth’s five oceans, with an area of approximately 5,427,000 square miles and an average depth of 3,410 feet. It is almost entirely bordered by land: Eurasia, North America, Greenland, and several islands. The Bering Strait connects the Arctic Ocean to the Pacific, and the Greenland and Labrador Seas join it to the Atlantic.

Map courtesy of Wikimedia Commons.

The presence of the continents and the shape of the ocean basin restrict the flow of water into and out of the Arctic Ocean. As a result, the Arctic Ocean is somewhat isolated from the rest of the oceanic circulation. Conditions in the Arctic are also unique to that region because of this isolation.

The North Atlantic current brings warmer water from the Atlantic, providing about 60 percent of the ocean’s inflow. Some water also moves into the ocean from the Pacific via the Bering Strait. Several rivers move freshwater into the ocean. The East Greenland and Labrador currents transport cold water from the Arctic Ocean into the Atlantic. Some water also moves into the Pacific via the Bering Strait.

Once a layer of ice is present, new ice forms on the underside of the existing floating ice layer. The ice actually forms an insulating blanket and at some point the water is no longer affected by the bitterly cold winter air of the northern latitudes.

The formation of sea ice also impacts the water’s salinity. As seawater freezes, salts and other substances in the water are squeezed out of the spaces between the water molecules. This means that salts are excluded from the ice, and that sea ice is actually more like freshwater ice (not as salty as seawater). As the process continues, small pockets develop in the ice, where the salt water becomes more and more concentrated into brine. This dense fluid moves downward through the ice and contributes to an increasingly dense layer of very cold water just below the ice. The cold, dense water sinks and moves south; in its place, warmer water moves north. This global transportation of water is known as thermohaline circulation – an important deep water current that connects all the ocean basins.

The temperature of the Arctic Ocean’s surface is fairly constant, around the freezing point of seawater. The relatively warm ocean water has a moderating effect, even when covered by ice. This is one reason why the Arctic does not experience the extreme temperatures of the Antarctic.

Like all other oceans, the Arctic Ocean plays an important role in the global climate. Cold air masses form over the Arctic Ocean and move toward the equator, mirroring the flow of cold water via outgoing currents. When cold air meets warmer air at the mid-latitudes, it causes storms and precipitation.

The Arctic Ocean is home to a surprising number of organisms – recent work for the Census of Marine Life estimates the number of species around 5,500. The Arctic marine food web begins with microscopic producers called phytoplankton. Some, known as ice algae, live on the underside of sea ice, absorbing solar radiation while benefitting from the ice’s insulation. Others float freely in the water. Diatoms are one important type of phytoplankton.

Marine diatoms seen through a microscope. These particular specimens were living between crystals of sea ice in McMurdo Sound, Antarctica. Photo credit: Prof. Gordon T. Taylor, Stony Brook University, USA.

Phytoplankton are eaten by zooplankton – tiny animals that drift in the water. One common zooplankton is the copepod, a small, shrimplike animal. Crustaceans, squid, jellyfish, and small fish make up the food web’s next tier. They are eaten in turn by larger predators such as large fish, seals, whales, walruses, and even polar bears, the top (apex) predator of the Arctic.

The Arctic Ocean remained largely unexplored and unknown until the late 1800s. The presence of sea ice prevented exploration – so much so that makers of navigational charts simply left the area blank! The first nautical crossing was made in 1896, and the first surface crossing (via dogsled) in 1969.

Today, climate change has caused a rapid decline in both the extent and thickness of Arctic sea ice. Summer sea ice has reduced by nearly 50 percent and ice that forms in winter is thinner and more susceptible to summer melting. Experts predict an ice-free Arctic Ocean as early as 2013, raising new commercial possibilities as well as the potential for international competition and conflict over predicted deposits of oil, natural gas, and minerals. Finally, the declining sea ice means that less of the sun’s energy is reflected back into space. The darker, open-ocean water absorbs more solar radiation, which is re-radiated back into the atmosphere as heat. The increased temperatures then cause further melting, forming a positive feedback loop.

At the opposite end of the earth is the Southern Ocean – another polar ocean, but one with very different characteristics.

THE SOUTHERN OCEAN: WATER SURROUNDING LAND

The Southern Ocean (also known as the Great Southern Ocean, the Antarctic Ocean, or the South Polar Ocean) is defined as the waters south of 60 degrees S latitude.

The Southern Ocean. Map courtesy of Wikimedia Commons.

Unlike other oceans, the Southern Ocean is not bordered or defined by land masses, and is not always named as a separate ocean in textbooks. However, oceanographers can distinguish between the characteristics of water in each of the world’s ocean basins. They find that the ocean surrounding Antarctica is very different from the water in the nearby basins.

Ocean currents play a large role in defining the Southern Ocean. The strong Antarctic Circumpolar Current circles the continent and effectively creates a barrier to the mixing of warmer water from the Atlantic, Indian, and Pacific Oceans into the Southern Ocean. On the whole, water of the Southern Ocean is colder, denser, and more saline than the waters of the surrounding oceans.

This computer-generated map shows the speed of the clockwise Antarctic Circumpolar Current. Slow moving water is shown in blue, and fast moving water is shown in dark red. Image courtesy of San Diego Supercomputer Center Multimedia Gallery.

Unlike the Arctic, which is fed by freshwater rivers, the Southern Ocean is fed by glacial melt and calving (breaking off of ice masses) from Antarctica’s ice sheets.

The ocean covers an area of 7,848,000 square miles and has an average depth between 13,000 and 16,000 feet. This makes it the fourth largest of earth’s five major oceans.

In winter, sea (or pack) ice forms on the ocean along the Antarctica coastline and extends out, almost doubling the size of the continent. The ice pack’s size fluctuates from approximately 1 million square miles in March to over 7 million square miles in September. The formation of sea ice causes an increase in the salinity of the water, making it denser. The denser water sinks, forming a deep current that eventually finds its way to the lowest places in the ocean basins. This deepwater circulation contributes to the general circulation of the oceans (and connects all the ocean basins). It takes decades to centuries for the water to travel the great distances involved.

Temperature and elevation differences between the ice sheets of Antarctica and the open ocean causes the formation of fierce katabatic (downward-moving) winds. Air currents and cyclonic storms travel eastward around the continent, mirroring the Antarctic Circumpolar Current and isolating the continent from warmer air masses from the north. The wind and ocean currents circle Antarctica and create a spinning “vortex” that prevents other water and air from mixing into this region. Therefore, moist area rarely travels inland over the continent, and snowfall is minimal. Antarctica is actually a desert, receiving less than 10 inches water equivalent of precipitation a year.

Once thought to be barren, the Southern Ocean is home to a large number of species. Expeditions are discovering many new species and current research estimates the number of named species at 7,500. Just as in the Arctic, phytoplankton form the base of the food web. These tiny, plantlike organisms drift through the ocean, using the sun’s energy to produce their own food. Zooplankton feed on the phytoplankton. Krill, a shrimplike crustacean, is plentiful in Antarctic waters. It provides an important food source for penguins, whales, and other marine species.

A Ross Sea food web. Image courtesy of David Allen, National Institute of Water and Atmospheric Research Ltd (NIWA).

The Southern Ocean and Antarctica are protected by a variety of agreements and treaties. The International Whaling Commission prohibits commercial whaling, the Convention for the Conservation of Antarctic Seals limits seal hunting, and the Convention on the Conservation of Antarctic Marine Living Resources regulates fishing. Finally, the Antarctic Treaty protects the waters and land south of 60 degrees S latitude.

Climate change is affecting the Southern Ocean in a number of ways. Climate change has increased calving and ice-sheet instability on the West Antarctic Ice Sheet. Rising temperatures have led to sea-ice decline, which has affected both the phytoplankton that serve as the base of Antarctic food webs and larger species that depend on the ice (Adelie penguins, Antarctic silverfish, and krill). Warming water also raises the potential for crabs and other predators to invade the Southern Ocean. These animals could reshape the existing food webs and potentially cause the elimination of some polar marine species. Penguins are competing more directly for food and breeding space, and the numbers and locations of penguin colonies are visibly shifting, as a result.

LINKS

Earth’s Ocean
Information about earth’s ocean system. Includes links to further information on a variety of topics.

Copyright May 2009 – The Ohio State University. This material is based upon work supported by the National Science Foundation under Grant No. 0733024. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.This work is licensed under an Attribution-ShareAlike 3.0 Unported Creative Commons license.